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Monday, November 13, 2017

14 inch JCV Television dead now repaired.

I customer came to my shop and asked if I can check out his
television with went dead yesterday while watching news.

He went back to his house and after ten minutes he was back
with the said television.

I plugged the TV via my series bulb to confirm what he said
and indeed the TV was dead.

Looking at the fuse I noticed it was not there, further
checking I noticed the TV was from another technician and after repairing
decided to use a just wire instead of actual fuse.

This friend is bad practice and should be discouraged
because a fuse in an important safety component.

Also I noticed the previous repairer has improvised the
supply with a universal power supply kit.

Since I noted he/she has not removed any components from the
primary supply I decided to repair the original supply and therefore I removed
the improvised power supply.

The first thing I replaced was the switching transistor and
with the switching transistor in circuit, I decided to apply the power again
using the serried bulb and the bulb went bright briefly and then off-This is
what is expected if the supply has no shorted component.

I decided to do some voltage testing on the primary side and
the best place to start was the voltage across the main capacitor and I got 290
vdc, this is not bad in our country because we use 240 volts and therefore
decided to continue with my troubleshooting.

I decided to start with testing for open resistors since I
was not expecting any shorted component according to my initial testing using
the series bulb.

From my experience the most common resistors to open are low
ohm/high wattage resistors and therefore they are big in size and I decided to
start with them.

As you can see these are very few on the power supply and I
tested them very quickly and got one which was open.

I also noted that it was connected to the emitter of the
Switching transistor. This resistor is called current sense resistor and very
important component on the power supply.

Usually if you find this resistor open expect the switching
transistor to have shorted also, therefore whenever you change the switching
transistor be sure to change this resistor also.

On the circuit board it was referenced as R510, kindly see
the diagram below.

After replacing this resistor I applied power again to see
the effect it has and still the set appeared dead.

I decided to do voltage testing again and this time I
decided to test the voltage on the Collector (middle) leg and the base (Left)
leg of the switching transistor and I got 290 at the middle leg and zero at the
base.

For this transistor to switch you must have these two
voltages, the one at the base is called start up voltage… Act like the actual
switch used to switch on electricity in your house.

To trace what is blocking this voltage I started testing the
voltage at the middle of the start up resistors…in this TV they were two and
testing at the middle of the two resistors I got 146 volts and the other side
of this resistor toward the base of the switching transistor I got zero volts.

I lifted one leg of the second resistor and the voltage
increased to 290 Volts.

With that I concluded that there must be a shorted component
on that path which I taking that voltage to the ground.

I suspected transistor V515 and V516 and I removed them
completely from the circuit and testing again I noted the same problem was
still there.

With that I decided to check out the diodes which common
failure mode is shorting.

Again the diode on the power supply are very few and after
testing a few in circuit with my digital meter set to diode range I noted one
which has reading on both sides of my meter probes.

I soldered one leg and re-tested and again got two reading,
with that I confirmed the diode reference D515 was indeed shorted and needed a
replaced.

I got one from a junk board around, I installed it and
applied power to the TV was up and running.